An Extreme High Temperature Event in Coastal East Antarctica

Over the last few years there has been a growing interest in ice loss from the coastal region of East Antarctica, with recent studies suggesting that the East Antarctic Ice Sheet has been a major contributor to sea level rise over the last four decades. While the greatest contribution to ice loss has come from basal melting of the ice shelves, satellite data have indicated that there has also been extensive surface melt during the summer months. With Antarctic temperatures expected to rise over the coming decades, it is anticipated that there will be greater surface melt and a growing threat to the ice shelves. It is therefore important to understand the mechanisms that lead to high temperatures in the East Antarctica coastal zone in order to predict how conditions may change in the future. This talk will describe the synoptic environment that led to record or near-record high temperatures in coastal East Antarctica in early December 1989. During this time, surface temperatures rose to 9.3 C at Mawson (the second highest in a record starting in 1954) and 10.1 C at Davis (the twelfth highest temperature in the record starting in 1957). While on average there is a minimum in depression activity in the circumpolar trough during the summer months, a number of deep storms do occur each summer, although these tend to be rather mobile, with a transient, moderate temperature signal at the coastal stations. Exceptionally, in early December 1989 a quasi-stationary, multi-centred depression became established for several days off the coast between approximately 50 and 100 E. This fed warm, humid air towards the coast for a number of days, with poleward thermal advection on the 300 K isentropic surface towards the high orography to the east of the Amery Ice shelf reaching an instantaneous maximum of 4.65 x 10**-4 K/sec at 6 UTC 4 December: within the top 1% of summer advection values at this location. The second phase of the warming started on 4 December when the downslope 10 m wind strength increases across 90-120 E between the low in the circumpolar trough and a summer high of record strength in the interior. Strong downslope flow is common during the winter months, but the speed of 15 m/s on 5 December was the highest December speed during the reanalysis period starting in 1979. The strong southerly flow drew down upper tropospheric air that warmed adiabatically just inland of the coast, further increasing the lower tropospheric temperatures. The coastal easterly winds were unusually deep and strong (30 m/s at 700 hPa on 4 December), and during 4 December the warm pool was advected towards the west, giving a short period of high temperature at coastal locations.